This invention relates to an elongated flexible treatment system for use in both industrial and medical applications and, more particularly, to a steering mechanism for catheters.
Catheters are well known devices used for insertion of devices to act upon or treat conditions of interest within cavities or the internal surfaces of cavities. While the present invention has application in many fields, it has particular relevance to the medical field, wherein flexible steerable catheters are employed to treat deep and circuitous passages of the human body.
A steerable catheter generally comprises an elongated insertion tube having a distal passively bendable portion and an optional rigid portion, a controllably bendable segment at the distal end of the passively bendable portion, a working channel for the passage of surgical instruments, fluids or aspirated materials, and a control member at the proximal end of the elongated tube for remotely steering the bendable segment. Usually, one or two pairs of control cables, depending on the number of planes of bending, extend through the controllably bendable section and the remainder of the insertion tube and connect with a steering control mechanism in the control member in order to remotely deflect the distal bendable section. One or both pairs of these cables are longitudinally displaced (pulled or pushed) to generate a bending moment in the steering section and deflect and steer the distal tip.
In a steerable catheter, opposing steering cables are displaced to deflect the distal tip. These cables are oppositely longitudinally displaced, that is, as one cable is pulled away from the bending section, the diametrically opposed cable moves toward the bending section to provide a moment about the tip. The cables are attached to the inner wall of the distal end of the catheter so that when pulled, the bending moment applied is proportional to the pulling force multiplied by the distance from the centerline of the section to which the cables attach.
There are two general methods of inserting catheters into the body: one in which an incision is made to access a cavity such as a blood vessel or the abdominal wall, and a second where the catheter is inserted through a natural aperture, such as the nose, mouth, urethra or rectum. A steerable catheter is typically inserted into a vessel or body cavity of a patient for treatment of tissues within the cavity. For example, a catheter can be inserted into the colon via the rectum, into a lung via the trachea or into the heart via the femoral artery. Because the various portals into the body comprise narrow, circuitous passageways, the steering section must be bendable rather precisely, and as close to the distal tip as possible, to navigate the passageway without damaging the patient's tissues.
The current trend in medicine is toward minimally invasive surgical techniques, and in applications such as neurosurgery, obstetrical/gynecological procedures, cardiovascular surgery, et al., there is a demand for smaller and smaller diameter catheters. To produce the necessary bending moments in a small diameter catheter, the force applied by the pulling cables becomes extreme, as the bending moment arm, or distance from the centerline of the section to which the cables attach, has been reduced. Unfortunately, it is not possible to simply increase the scale of the pulling cables to compensate for the increased forces. If the cables were allowed to be increased to take added load, the available space within the smaller diameter insertion tube would be reduced, resulting in less room for the working channels.
There are recurring problems which result when excessive stresses are applied to the operating cables by the control mechanism. In an extreme situation, the cable can break or, in a less extreme situation, the cable can be permanently stretched. In the former instance, the catheter is rendered useless until the cable has been replaced. In the latter instance, the catheter loses a portion of its original deflection capability, making it necessary to take up the slack of the stretched cable or recalibrate the instrument. Also, if the cables on one side stretch, the deflectable portion of the catheter will not return to a straightened form when it is relaxed. In the case of reusable catheters, it is necessary to open the instrument, usually at the factory or at a well-equipped service center, and perform the necessary servicing to return the instrument to its nominal operating condition. Of course, if a problem arises in the midst of a surgical procedure, the surgery may be interrupted and delayed for critical minutes while a working catheter is re-inserted into the patient.
To repeatedly and efficiently cause a particular deflection angle in deflectable catheters, the stress developed in the control cables must be less than the yield stress of the wire. To ensure that this criteria is satisfied, some devices incorporate slip clutches, force distributing or force limiting systems to avoid overly stressing the control wires. Devices of this kind are shown in U.S. Pat. Nos. 4,762,118, 4,762,119 and 4,787,369. Other devices, such as in U.S. Pat. No. 4,688,555, include a cable tensioner to guard against high loads and take up cable slack.
The stiffness of the controllably bendable section and working channel directly affects the amount of force necessary to deflect the distal tip. A balance has been pursued by numerous designers and inventors whereby the material properties, configurations and dimensions of the component parts of catheters have been adjusted to keep the induced wire stress below the yield stress of the wire for given deflection angles. These endeavors have seen the development of highly elastic polymers, the adoption of unique tubular geometric profiles, e.g., notched tubes, and the replacement of deformable, i.e., elastic, materials with articulating disks or vertebrae.
It appears that with every new breakthrough, a demand for deflectable tubular devices of even smaller diameter presents itself. With the development of novel tube or conduit constructions and the use of highly elastic materials, extrapolation of existing designs to even smaller catheters has resulted in a tremendous amount of stress being placed upon the actuating wires and the connection of these wires to the tubular device when significant deflection is required. This stress has resulted in the frequent occurrence of device failure due to either load wire fatigue, stretching and breakage or to bond failures between the load wire and the tubular device. As catheters become smaller in cross section, a practical limit is reached whereby the only way to insure that the wires do not fail is to reduce the tension on the wires, thereby reducing the maximum angle of deflection.
Therefore, there is a need for a steering mechanism for minimally invasive catheters which reduces the possibility of cable fatigue, stretching or breakage while retaining desirable deflection capabilities.